The present invention relates generally to the art of paper and document handling for computer controlled printers, document scanners, automatic document feeders and other applications and, more particularly, to apparatus capable of separating single sheets from a stack thereof.
Automatic document processing apparatus such as scanners and desktop printers typically includes a shelf or tray for holding a stack of one or more sheets of documents to be scanned or blank paper or other media to be fed, one sheet at a time, to the other portions of the document processing apparatus. An example of a typical prior art single sheet separator is schematically shown in FIG. 1 in side elevation view for handling a stack of paper or printed documents to be processed. As shown, the stack S is horizontally oriented but persons skilled in the art will appreciate that the paper stack or holding tray may be inclined from the horizontal so as to feed sheets from the top of the stack to the single sheet separator mechanism with the assist of gravity. The prior art single sheet separator comprises a single sheet separator roller 10 rotatably mounted near the end of a support arm 20 which in turn is pivotally connected to support structure 30 such as the chassis of the document handling apparatus. The separator roller 10 has an arcuate surface textured for frictionally contacting the leading edge upper surface of the topmost one of the stack of sheets for moving the sheet to the left into engagement with an upwardly inclined multi-sheet separator wall 32. A multi-sheet retard pad 40 which has a flat surface having a coefficient of friction designed for holding the lowermost sheet in the stack to minimize multi-sheet picks of the last few sheets is also provided. In other words, the coefficients of friction of the retard pad 40 and the separator roller 10 are selected, as is well known in the art, in accord with the type and surface coefficient of friction of the individual media sheets to be handled which may be paper, vellum, transparencies, or other types of media. Typically, the stack S is supported on a support surface or tray 50. The pivot point 22 of the support arm 20 (which may be two or more spaced arms respectively positioned at opposite axial ends of the separator roller or rollers) may be downwardly biased by a spring 60 which provides a known amount of force urging the separator roller 10 toward the stack S. Rotary power may be imparted to the separator roller through a train of gears 12, 14, 16, 18 as shown or through any other suitable form of power transmission. It will be noted that the pivot point of the support arm is necessarily located above the stack of sheets to be handled due to space limitations and the usual configuration of the document handling apparatus. The single sheet retard pad 40 is ordinarily angled downwardly (to the right as shown) at an angle a from the horizontal to ensure that the leading edge of the last sheet or last few sheets in the stack of media does not engage the side edge of the retard pad 40.
Due to differences in the coefficients of friction of the sheets of media and of the media engagement surface of the retard pad, excessively high separator roller drive torque loads are experienced when the last sheet of media has been discharged from the system. This high torque condition is known to frequently cause irrecoverable printer errors. Potentially, the applied drive torque is no longer adequate to rotate the separator roller causing potentially self-locking of the separator roller.
A further problem is excessive high gain, which is the ratio of the total frictional force opposing movement of the sheet divided by that portion of the frictional force which is imparted by rotation of the separator roller 10, as will be explained. High gain results in unpredictable forces applied normal to the surface of the sheets of media and resultant unpredictable sheet separation performance. High gain is also known to cause great variation in the normal force exerted by the separator roller downwardly toward the retard pad 40 depending on the coefficient of friction between the separator roller and the type of media sheets in use.
It is, accordingly, an objective of the present invention to overcome the above noted problems in the typical prior art structures.
The present invention therefore provides a single sheet feeder for moving individual sheets from the top of a stack thereof comprising: a support surface for said stack, a driven sheet separator roller having an annular sheet engaging friction surface, a last sheet retard pad in said support surface in opposed relationship to said friction surface for retarding movement of the bottom sheet in the stack to minimize multi-sheet picks, and a roller support pivotally connected to chassis structure, said separator roller being rotatably mounted on said roller support and said separator roller being moveable and biased toward said support surface and said sheet retard pad, said sheet retard pad having a sheet engaging friction surface including a portion downwardly inclined from the direction of movement of sheets from said stack, said sheet separator roller being arcuately moveable toward said downwardly inclined portion of said retard pad.
The present invention further provides a single sheet feeder for moving individual sheets of media from the top of a stack thereof comprising: chassis structure including a media stack support having a stack support surface, a driven sheet separator roller having an annular sheet engaging friction surface, a last sheet retard pad recessed in said support surface and having a sheet engaging friction surface forming part of said support surface positioned proximate said separator roller for retarding movement of the bottom sheet in the stack to minimize multi-sheet picks, and a roller support pivotally connected to chassis structure, said separator roller being rotatably mounted on said roller support and said separator roller being moveable and biased toward said support surface, said stack support surface having a roller reception recess proximate said retard pad, said sheet separator roller being arcuately moveable toward said recess in said stack support surface, and arcuate movement of said roller support and roller being terminated before said roller engages said stack support surface in said recess by contact of said roller support with said chassis structure.